1. Field of the Invention
The present invention relates generally to integrated circuits for driving an active or passive matrix liquid crystal display (LCD) or the like, and more particularly, to an integrated circuit which provides a digital-to-analog conversion scheme for efficiently converting digital signals corresponding to gray shades into analog signals for driving columns of the LCD display matrix.
2. Description of the Background Art
Active matrix LCD displays are used today in a variety of products, including hand-held games, hand-held computers, and laptop/notebook computers. These displays are available in both gray-scale and color forms, and are typically arranged as a matrix of intersecting rows and columns. The intersection of each row and column forms a pixel, or dot, the density and/or color of which can be varied in accordance with the voltage applied thereto in order to define the gray shades of the liquid crystal display. These various voltages produce the different shades of color on the display, and are normally referred to as "shades of gray" even when speaking of a color display.
It is known to control the image displayed on the screen by individually selecting one row of the display at a time, and applying control voltages to each column of the selected row. This process is carried out for each individual row of the screen. After each row has been selected, the process is repeated to refresh and/or update the displayed image.
LCD displays used in computer screens require a relatively large number of such column driver outputs. Color displays typically require three times as many column drivers as conventional "monochrome" LCD displays; such color displays usually require three columns per pixel, one for each of the three primary colors to be displayed. Thus, a typical VGA (480 rows.times.640 columns) color liquid crystal display includes 640.times.3, or 1,920 column lines which must be driven by a like number of column driver outputs.
The column driver circuitry is typically formed upon monolithic integrated circuits. Assuming that an integrated circuit can be provided with 192 column output drivers, then a color VGA display screen requires 10 of such integrated circuits (10.times.192=1,920). Due to the relatively large number of such column driver integrated circuits that are required by such a color VGA display screen, the cost of such column driver integrated circuits can greatly influence the overall cost of the display.
Integrated circuits which serve as column drivers, also known as source drivers, for active matrix LCD displays must generate different output voltages to define the various "gray shades" on a Liquid Crystal Display. These varying analog output voltages vary the shade of the color that is displayed at a particular point, or pixel, on the display. The computer, or other device used to control the video display, generates digital values that roughly correspond to the magnitude of the voltage to be applied at each pixel, and hence, the particular shade of color to be displayed at each pixel. However, the task of storing such digital values, converting such digital values into actual analog voltages, and driving the analog voltages onto the columns of the display matrix in the correct timing sequence is left to the column driver integrated circuits.
Known column driver integrated circuits typically require that the series of analog voltages corresponding to the various shades of gray be developed remotely from the integrated circuit. For example, one known integrated circuit column driver includes sixteen input terminals for receiving a series of sixteen different analog voltages that are generated off the chip. On board the integrated circuit, a 16:1 analog multiplexer is provided at the output of each column driver to select one of sixteen analog voltages to the output; thus, the sixteen analog voltages are simply passed directly by the analog multiplexer to the column driver output.
The known system described in the preceding paragraph is limited to only sixteen shades of gray for each color in the display. This method works extremely well for low numbers of voltages (2 to 16), but the number of required input voltages increases linearly with the number of output voltages needed. For high gray shade displays, which may require 256 or more distinct analog voltages, this method requires an equal number of input voltages and a great deal of system level space due to the routing of these voltages. In addition, these voltages must be generated externally, requiring another integrated circuit which must produce them. If more than sixteen shades are desired, the number of analog input voltages, and the size of each multiplexer provided at each output, must be increased. The number of input terminals of integrated circuit packages must be kept to a minimum in order to allow more of the package pins to be used for serving as column output drivers. Thus, the need to bring on-board, route, and multiplex a relatively large number of analog voltages is a distinct disadvantage.
Another technique is also known in the art for supplying a series of analog voltages to a column driver integrated circuit, but without requiring the dedication of a large number of input terminals to bring on-board the integrated circuit the array of analog voltages. In this second type of known integrated circuit, an external voltage generator provides only one analog voltage at a time; the external analog voltage generator is sequentially switched between a number of different analog voltages. For each column driven by the integrated circuit, the digital value corresponding to the desired shade of gray for that column is sequentially provided to the external voltage generator. Thus, the digital value for the first column is supplied to the external voltage generator, and the corresponding analog voltage is generated and supplied to the integrated circuit. A sample and hold circuit associated with the column driver output for the first column is enabled to sample the externally generated analog voltage. Next, the digital value for the second column is supplied to the external voltage generator, and the corresponding analog voltage is generated, and supplied to the integrated circuit. A sample and hold circuit associated with the column driver output for the second column is then enabled to retain the correct analog value for the second column. This process is repeated sequentially for each of the columns driven by the integrated circuit until the analog voltages for all 192 columns driven by one integrated circuit have been sampled and held.
In this second method described in the preceding paragraph, each output would sample the voltage and then hold it, using an amplifier to drive the output. This method requires that 1,920 sequential samples (192 columns per chip times 10 chips) be made during a single row time in a typical color display. Because of the number of column drivers, the external voltage generator must be able to switch rapidly in order to sample each column driver output in a given amount of time. The time required can be reduced somewhat by providing additional analog voltage inputs; typically, there are a total of at least three voltage inputs containing the red, green, and blue voltages for color optimization on the display. When large displays with high numbers of gray shades are needed, this method requires extremely high speed, high power external digital-to-analog converter and associated amplifiers to drive these voltage inputs. However, external voltage generators capable of maintaining such rapid switching rates are more complex and expensive.
Accordingly, it is an object of the present invention to provide an integrated circuit column driver chip that is capable of generating a series analog output voltages, corresponding to varying shades of gray on the LCD display, locally on-board the integrated circuit column driver chip to thereby minimize the number of analog voltage input terminals, and to eliminate the need to generate such series of analog voltages externally from the integrated circuit.
It is another object of the present invention to provide such an integrated circuit which eliminates the need for a rapidly switching external analog voltage generator in order to generate the required series of analog voltages, corresponding to the varying shades of gray on the LCD display.
A further object of the present invention is to reduce the number of sampling cycles required on-board the column driver integrated circuit while still allowing all column driver outputs to sample their corresponding analog output voltage.
A still further object of the present invention is to decrease the number of such sampling cycles to a number that is less than the number of different shades of gray to be displayed upon the LCD display.
A yet further object of the present invention is to provide an analog voltage generator on-board such a column driver integrated circuit for generating a relatively large series of analog voltages corresponding to varying shades of gray on the LCD display.
Still another object of the present invention is to provide such a series of analog voltages which vary in a non-linear fashion when compared with the digital representations of the shades of gray to be displayed upon the LCD display.
These and other objects of the present invention will become more apparent to those skilled in the art as the description of the present invention proceeds.